Building Science Fundamentals
Hygrothermics and the principle of continuous control layers
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Building Science Foundation:The Core of Durability(and risk reduction)
Full Disclosure
Premises
• Things get wet, heat dries them out.
• Energy efficiency measures reduce heat loss.
• Energy & moisture must be managed with equal intensity.
• Do this, and buildings are durable; don’t and not-so-much…
Just Three Things
• How does heat get around?
• How does water get around?
• How are the two related?
Heat moves in 3 ways
• Radiation
• Conduction
• Convection
• 25% - 40% of heat loss in a typical home is air leakage.
Combining IR & Blower Door
Blower door off
Blower door on (-50 Pa)
ice vapor
water
“film”
Moisture Control in Buildings
sublimate
frost
*with this “new” one important because of the porous nature of many building materials
Dealing With Water
Moisture Flows – 4 Ways:Gravity – Priority #1
•Tempered by:• Lateral cohesive movement
• Wind-driven rain
•Can involve LOTS of water—from an outside or inside source
Moisture Flows – 4 Ways:Capillarity – Priority #2
Capillary Breaks
18.0% 12.8%
How many ways can a building get wet?Moisture Flows – 4 Ways:Air Leakage – Priority #3
How many ways can a building get wet?
How many ways can a building get wet?
Air SealMoisture Flows – 4 Ways:Air Leakage – Priority #3
Air leakage driving forces
Stack effect
Wind
Fans (exhaust)
Moisture Flows – 4 Ways:Diffusion– Priority #4
Russ Chapman, Sierra Pacific Windows, SEON
Special situations: humidifiers
Scroll wheel
Steam
Household Sources of Moisture• Sources - Household Moisture
Source Quantity (pints)
Showering .5 (5 - min shower)
Clothes drying 4 - 6/load
Cooking (dinner) 1.2 (+1.5 gas)
5 house plants 1/day
1 cord “green” wood 600 - 800/season
4 people .5/hour
Building materials 6 - 17/day
Ground moisture 0 - 100/day
Source: Minnesota Extension Service (also, see GBA blog…)
https://www.greenbuildingadvisor.com/article/moisture-sources-relative-humidity-and-mold
Prioritizing moisture worries…
• Bulk water (liquid)
• Capillary water (liquid)
• Air-transported moisture (vapor)
• Diffusion (vapor)
Continuous control layers
• 1 – Water• Bulk
• Capillary
• 2 – Vapor• Air leakage
• Diffusion (more about drying than wetting)
• 3 – Thermal• Bridges
• Cavities
The “Pen” Test
2010 © Building Green LLC
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Continuous bulk water management
Barrier vs Screen Assemblies
• Barrier: Cladding is drainage plane• Sealants are key to “face-sealing” penetrations
• Sealants are exposed
• Sealants can and must be inspected, repaired and replaced
• Screen: Concealed drainage plane is primary line of defense
• Sealants and PSA tapes are key to connecting WRB to flashings at openings and penetrations
• Sealants and tapes are buried
• Sealants and tapes cannot (easily) be inspected, repaired and replaced
• Sealants and tapes must last life of assembly
EPA
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Principles of bulk water management
• Physics first• Weatherlap first
• Mechanically support
• Chemistry second• Liquid sealants
• PSA tapes
• Anything not vertical is a “roof”
Continuous air control layer
https://www.buildinggreen.com/sites/default/files/usace_airbarriercontinuity.pdf
Air Barrier Continuity: A Quick Guide to Sealing Air Leakage Pathways in Buildings
Air Barrier Continuity: A Quick Guide to Sealing Air Leakage Pathways in Buildings
Air Barrier Continuity: A Quick Guide to Sealing Air Leakage Pathways in Buildings
Air Barrier Continuity: A Quick Guide to Sealing Air Leakage Pathways in Buildings
Air Barrier Continuity: A Quick Guide to Sealing Air Leakage Pathways in Buildings
Vestibule
Elevator Elevator
Elevator Elevator
Utility chase
Mail chute
Corridor Corridor StairsStairs
Trash chute
Laundry chute
Pressure boundary
Pressure boundary
Primary Air Barrier Components
• Field• Sheet good/membrane
• Rigid sheathing
• Fluid-Applied
• Connection• PSA tapes
• Mesh/Mastic
• Systems
• Field examples• SBPO (Spun-bonded
polyolefin)/Henry Blueskin
• ZIP system
• DOWSIL
• Connection examples• Acrylic, Butyl, Modified bitumen
• System examples• Huber 4-component ZIP system
EPA
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Airtightness proof positive: blower door test
Thermal Control Layer
• Exterior vs. cavity insulation
• Connection to air control layer
• Thermal breaks vs. thermal bridges
• Opaque assemblies vs. glazing (special case)
Thermal Bridges
Thermal bridges…
Breaking Thermal Bridges• Materials
• Stainless steel
• Fiberglass
• Poylester-reinforced nylon
• Intermittent rather than continuous structural support
• Maintain plane
• Ladder constructions stuck on later, when you can
• Push spaces in or pull them out of the condition boundary
EPA
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Vapor Control/Profile (not vapor retarder)
How many discrete components or layers make up a “typical” building assembly?
In order for this assembly to dry, which components layers do you need to know the vapor permeability of?
Vapor Profile:If/When a building assembly gets wet, will/how will it dry?
Vapor Profile Considerations
• The weakest form of wetting (diffusion) is our last resort for drying (diffusion).
• So, we need to know vapor permeance of our enclosure materials not because assemblies get wet by diffusion, but because that is how, finally, they will dry (or not).
• To determine direction and extent of an assembly’s drying potential, need to know vapor permeability of ALL layers of your assemblies.
1. Determine vapor permeability of each component of assembly
2. Categorize each component (Class I, II, III, vapor open)
A. Class I: < 0.1 perms
B. Class II: 0.1 – 1 perms
C. Class III: 1 – 10 perms
D. “Vapor open:” > 10 perms
3. Assess direction and extent of vapor drive: interior/exterior temperature difference, interior/exterior relative humidities(remember always high to low)
4. Identify/assess drying direction & potential
Vapor ProfileVapor Profile
Representative Vapor Permeability Info
Material Dry Cup Wet Cup Comments
Plywood .75 3.5 Semi-permeable
OSB .75 2 Semi-
Fiberboard (AI) 14.5 15 Permeable
Thermo Ply 0.5 0.6 impermeable
XPS 1 1 Semi (but with skin, im-)
EPS 5 5 Semi-
6-mil poly .06 .06 Impermeable
Kraft paper 1 5 - 10 Semi- (variable)
MemBrain™ 1 10+ Variable, by design
Tyvek® 14 ? permeable
Latex paint
(primer + 1
coat)
3.6 6 Semi-
BMPT
Hold on: Exactly what is a perm?
• 1 grain H2O moving through
• 1 square foot of building material
• In 1 hour
• At 1 inch of Hg pressure difference
• 7000 grains H2O = 1 lb H2O
• 1 inch Hg vapor pressure difference?
Image credit: danieloverbey.blogspot.com
Vapor Profile: Example 1
1. Latex paint - 17 perms
2. Wall board - 40
3. Cellulose - 75
4. XPS - 1
5. Air space - “300”
6. Wood siding - “35”
7. Oil-based paint - 0.6
Least permeable - 4. XPS
1
4
25
3
6
7
Vapor Profile: Example 2
1. Vinyl wallpaper - < 0.1 perms
2. Wall board - 40
3. Cellulose - 75
4. Foil-faced polyiso - > 0.1
5. Vinyl siding - “60”
Least permeable - 1 & 4
1
4
2
5
3
Principal Resource – Vapor Profile:GBA Building Science blog…
Areas of Focus for the BE Protection Systems
• Below-grade walls, Above-Grade walls, Roofs
• Underlayments
• Claddings
• Penetrations (connecting/weatherlapping flashings to WRB)
• Margins
• Transitions
WUFI – hygrothermal modeling
Building science summary• Manage energy & moisture with equal intensity.
• Manage all types of moisture but generally in this order of priority:• Bulk water
• Capillary water
• Air-transported vapor
• Vapor diffusion
• Who cares about this stuff? Believe or not, building owners do…
TABLE R702.7.1 CLASS III VAPOR RETARDERS
2015 Model Energy Code: Insulation, venting, VRs?
High Performance HVAC(D?)
Latent Loads…
Latent Loads Increase as a Percentage
of Total Loads at Part-Load Conditions.
Summary – Action Items
Worry/work in this order:
1. Bulk water management
2. Capillary breaks
3. Air leakage
4. Vapor diffusion (INCLUDING interior sources of moisture from those darn occupants…)
5. Manage each with continuous control layers
WETTING: DRYING:
YOURBUILDING
- Bulk water- Capillary water- Air-transported
- Diffusion
- Free drainage- Cap break
- Convection- Diffusion
Hygrothermal Balance